friend requests and privacy settings

My privacy settings show that I could either allow everyone to send me friend requests, or limit them to friends of friends. Obviously, many famous people tend to use the latter because they’d be swamped with friend requests otherwise.

I happen to be friends with a well-known New York Times journalist, and he is friends with several public figures (including Mark Zuckerberg and Wikipedia’s founder). However, I still don’t see the “Add friend” button for some of those people, even though I should be able to send them friend requests due to the second-degree connection (not that I’d want to anyway).

So I’m just curious, is there a third privacy setting that’s available to certain users? Or does it have something to do with the 5,000-friend limit?